510 
PEOFESSOE JEVONS ON THE MECHANICAL 
the letter pins, representing the terms of the combination, and varied in each pair of rods 
to correspond with the letters of the abecedarium. On examining fig. 1, it will be appa- 
rent that the pins are distributed in a negative manner ; that is to say, it is the absence 
of a pin in the space A, and its presence in the space a, which constitutes the rod a 
representative of the term A. The rods belonging to the combination A b C cl, for 
instance, have pins in the spaces belonging to the letters a, B, c, 1). 
3G. The key board of the instrument is shown in fig. 4, where are seen two sets of 
term or letter keys, marked A, a, B, b, C, c , D, d, separated by a key marked Copula — Is. 
The letter keys on the left belong to the subject of a proposition, those on the right to 
the predicate, and on either side just beyond the letter keys is a Conjunction key, appro- 
priated to the disjunctive conjunction or, according as it occurs in the subject or pre- 
dicate. The last key on the right hand is marked Full Stop, and is to be pressed at 
the end of each proposition, where the full stop is properly placed, On the extreme 
left, lastly, is a key marked Finis, which is used to terminate one problem and prepare 
the machine for a new one. 
37. In order to gain a clear comprehension of the action of these keys, we must now 
turn to fig. 2, where all the levers are shown in position, only three of them being in- 
serted in fig. 3, and to figs. 6-13 (Plate XXXIV.), which represent, in the full natural 
size, the relative positions of each kind of lever with regard to the pins in every possible 
position of the rods. 
If the subject key A be pressed it actuates the lever A at the back of the machine; 
and supposing all the rods to be in their proper initial positions, it moves upwards, as in 
fig. 6, all the back a rods through exactly half an inch, the front rods connected with them 
of course falling through half an inch. All the a combinations are thus caused to disap- 
pear from the abecedarium ; but as the A rods have no pins opposite to the A lever, they 
will remain unmoved, and continue visible. Thus the pressure of the A key effects the 
selection of the class A of the conceivable combinations. Each subject letter key simi- 
larly acts upon a lever at the back ; and should several of them be pressed, either simul- 
taneously or in succession, the combinations containing the corresponding letters will 
be selected. 
38. Each predicate letter key is connected with a lever in the front of the machine, and 
when pressed the effect is exactly the same as that of a subject key, but in the opposite 
direction (fig. 11). If the B predicate key be pressed it raises through half an inch all 
the front rods which happen to have corresponding pins, and to be in the initial position. 
The back rods will at the same time fall, and the combinations containing b will disap- 
pear from the abecedarium, but in the opposite direction. 
39. It is now necessary to explain that each rod has four possible positions fully indi- 
cated in the figs. 6-13. The first of these positions is the neutral or initial position, 
in which the letters are visible in the abecedarium, and the letter pins are opposite 
letter levers so as to be acted upon by them. The second position is that into which a rod 
is thrown by a subject key; the third position lies in the opposite direction, and is that 
